Carbon-based materials, in general, enjoy wide utility due to their unique physical and chemical properties. Recent attention has turned to the use of elongated carbon-based structures, such as carbon fullerene filaments, carbon tubes, and in particular nanosized carbon structures. It has been shown that these new structures impart high strength, low weight, stability, flexibility, good heat and electrical conductance, and a large surface area relative to volume for a variety of applications, such as high-strength fibers, threads, yarns, fabrics, and reinforcement for composites, e.g., nanotube-reinforced epoxy structures.
Of growing commercial interest is the use of single-wall carbon nanotubes to store hydrogen gas, especially for hydrogen-powered fuel cells. Other applications for carbon fibers and/or nanotube materials include catalyst supports, materials for manufacturing devices, such as a tip for scanning electron microscopes, electron field emitters, capacitors, membranes for filtration devices as well as materials for batteries. In short, interest in nanotube technology arises from the very high strength, and electrical and thermo-conductive properties of individual nanotubes.
Finer than carbon fibers, the material with one micron or smaller of diameter is generally called carbon nanotubes and distinguished from the carbon fibers, although no clear line can be run between the both types of carbon fibers. By a narrow definition, the material, of which carbon faces with hexagon meshes are almost parallel to the axis of the tube, is called a carbon nanotube and even a variant of the carbon nanotube, around which amorphous carbon and metal or its catalyst surrounds, is included in the carbon nanotube. (Note that with respect to the present invention, this narrow definition is applied to the carbon nanotube.).
Usually, the narrowly-defined carbon nanotubes are further classified into two types: carbon nanotubes having a structure with a single hexagon-connected carbon-mesh in a tube form are called single-wall nanotubes (hereafter, simply referred to as “SWNT”); the carbon nanotubes made of multi-layer hexagon-connected carbon tubes are called multi-wall nanotubes (hereafter, simply referred to as “MWNT”). When grown from a substantially flat substantially planar surface (e.g., a nanoporous surface coated with an iron-oxide catalyst), the typical result is MWNTs. When grown in a dense aligned structure, the parallel nanotubes somewhat resemble a forest, and are referred to generally as a nanotube forest or more specifically as an MWNT forest. The type of carbon nanotubes may be determined by how they are synthesized and the parameters used to some degree, but production of purely one type of the carbon nanotubes has not yet been achieved.
U.S. Pat. No. 6,232,706 entitled “Self-oriented bundles of carbon nanotubes and method of making same” issued May 15, 2001 to Hongjai Dai et al. is incorporated herein by reference. Dai et al. describe a method of making bundles of aligned carbon nanotubes (e.g., for a field-emission device, such as a plasma TV screen) on a porous surface of a substrate, the method comprising the steps of: a) depositing a catalyst material on the porous surface of the substrate and patterning the catalyst material such that one or more patterned regions are produced; and b) exposing the catalyst material to a carbon-containing gas at an elevated temperature such that one or more bundles of parallel carbon nanotubes grow from the one or more patterned regions in a direction substantially perpendicular to the substrate.
Nanotube forests can be combined together to form structures possessing extreme strength characteristics. These strength characteristics, however, are limited by impurities in the structures themselves arising during the manufacturing process, and/or from the design of the structures such that the maximum possible surface-to-volume ratio is not used by the structure. The present invention addresses these and related issues.